I always thought the way to do this was via things in game already.
E.G. for Mercs you can pick from Malta, Nemesis, longbow, Arachnos, PPD, RIP; any number of other groups. And they don't have to use those groups weapons, it would be nice but even just the character model swap would be good. For bots you again have options like clockwork, new clockwork, malta bots, Nemesis bots...
Basically any enemy group with minion LT, boss that remotely fits a given theme could be setup as a power customization you pick just like bright/dark color ways.